High power radio frequency and microwave generators are known and it is desirable with such generators to be able to provide a facility for quickly changing the frequency of the output radio frequency signal. Such generators are required to produce high power pulses of radio frequency with peak powers, for example, in excess of 100 kW. For convenience the term radio frequency as used in this specification is intended to mean radiation in the high frequency, very high frequency, ultra high frequency and microwave regions of the electromagnetic spectrum.
It is desirable to be able to adjust the output frequency of a high power radio frequency generator rapidly and over a relatively large tuning range. For example it is useful to be able to adjust the frequency of the output signal in a time of the order of 0.1 millisecond to 1 millisecond, which allows frequency variation at pulse repetition frequencies of 1 kHz to 10 kHz.
Examples of known high power radio frequency generators are magnetrons, klystrons and travelling wave tubes (TWTs). These devices generally operate by generating a beam of electrons which passes through a periodic mechanical structure designed such that the electron beam interacts with the mechanical structure so that energy is extracted from the electron beam to produce a radio frequency or microwave field inside the mechanical structure which is normally a cavity. The characteristics of the radio frequency output from such known devices are fixed partly by the design of the mechanical structure and the characteristics of the electron beam and the structure together determine the range of radio frequency oscillation frequencies which can be supported and the quality factor of the cavity. Such devices may act as amplifiers for a small radio frequency signal.
High power oscillators such as magnetrons have been developed to produce output powers up to a few megawatts with tuning ranges of about 10 percent. The radio frequency output of such magnetrons is tuned by mechanical adjustment of the cavity and very high power magnetrons usually have relatively small tuning ranges. Specialised magnetrons which include rapidly rotating disks are able rapidly to adjust the radio frequency output frequency from pulse to pulse but are limited to relatively low output powers of less than 1 megawatt with constrained frequency agility resulting from fixed cycling patterns.
Klystron amplifiers may be used to produce high radio frequency output powers with the frequency being tuneable by variation of the frequency of the low power radio frequency input signal. As the peak power of a klystron is increased the band width is reduced and the effective tuning range is reduced accordingly.
Travelling Wave Tube amplifiers have been developed with band widths of one octave but this restricts the operating power to relatively low values of typically less than 100 kW. At higher power levels the band width is reduced to about 10 to 15% at 1 megawatt power.
Thus at power levels of more than a few megawatts conventional electron beam radio frequency generators have small band widths and small tuning ranges. High power magnetrons require mechanical adjustments to the cavity to adjust the radio frequency output frequency. Amplifiers such as klystrons and TWTs have severely restricted band widths at high power levels with correspondingly reduced frequency change agility.